分形理论与矩法估计联合的页岩孔隙结构表征

Characterization of shale pore structure based on fractal theory and moment estimation

页岩孔隙结构特征研究对页岩油气的评价与开发具有重要意义。选取川东南地区下志留统龙马溪组页岩为载体,采用FHH分形模型计算了页岩孔隙分形维数,以地质混合经验分布与矩法估计原理为基础,计算了孔隙结构特征参数,并进一步探索了分形理论与矩法估计对页岩孔隙结构表征的适用性。研究发现,页岩孔隙具有明显的分形维数,分形维数大,说明页岩孔隙系统复杂多变,非均质性较强。分形维数与页岩孔隙比表面积正相关,与孔径均值、标准差、变异系数和歪度负相关,即孔径均值越小,微孔所占比例越大,孔隙间连通性变差,反映孔隙结构较差且复杂程度高,但孔内比表面积变大,对气体的吸附能力变强,分形维数变大;反之亦然。页岩分形维数和孔隙结构特征参数与孔隙形态特征表现出良好的吻合性,即H2型墨水瓶孔分形维数大,标准差、变异系数和歪度小;H3型平行板状裂缝型孔隙的分形维数相对较小,标准差、变异系数和歪度较大。分形维数是页岩对气体的吸附和储运能力强弱、孔隙形态特征的反映。

Research on the characteristics of shale pore structure is of great significance to the evaluation and development of shale oil and gas. Taking the shale of the Longmaxi Formation of the Lower Silurian in the Southeast Sichuan Basin as the carrier,the fractal dimension of the shale pore was calculated by the FHH fractal model. Based on the principle of geological mixing empirical distribution and moment estimation,the characteristic parameters of pore structure were calculated,and the applicability of fractal theory and moment estimation to the characterization of shale pore structure was further explored. The study indicates that shale pores have an obvious fractal dimension,and the fractal dimension is large,indicating that the shale pore system is complex and changeable and has strong heterogeneity. The fractal dimension is positively correlated with the specific surface area of shale pores,and negatively correlated with the mean value of pore diameter,standard deviation,coefficient of variation,and skewness. That is,the smaller the mean value of pore diameter,the larger the proportion of micropores,and the worse the connectivity between pores,reflecting the poor pore structure and high complexity,but as the specific surface area in pores becomes larger,the adsorption capacity of gas becomes stronger,and the fractal dimension becomes larger. The fractal dimension and pore structure characteristic parameters of shale are in good agreement with the pore morphological characteristics,that is,the pore fractal dimension of the H2 ink bottle has a larger fractal dimension,small standard deviation,coefficient of variation,and skewness. The fractal dimension of the H3 type parallel-plate fractured pore is relatively small,and the standard deviation,coefficient of variation,and skewness are large. The fractal dimension is a reflection of shale gas adsorption,storage,transportation capacity,and pore morphology characteristics.